a. Field of the Invention
The present invention relates to a method of manufacturing sulfuric acid free of heavy metals, particularly mercury.
B. Description of the Prior Art
According to the general method of manufacturing sulfuric acid by the contact process, the gas coming out of the roasting furnace is refined by flowing it through a waste heat boiler, cyclone, dust Cottrell, cooling tower, washing tower, gas cooler and mist Cottrell, in that order, and the thus refined gas is deprived of its moisture by means of a drying tower, and is converted into SO.sub.3 through a group of convertors, and is then absorbed in sulfuric acid within the absorption tower to become a 98.degree. sulfuric acid.
In manufacturing sulfuric acid nowadays, the sulfides of nonferrous metals such as copper, lead, zinc, etc. are often employed as the raw material. Such a material ore sometimes contains substances which are hard to remove by the conventional apparatus, such as fluorine compound, mercury, etc. as impurities.
Particularly, mercury cannot be removed by the aforesaid refining apparatuses: it is apt to be introduced into the drying tower and mixed with the sulfuric acid, though in a very small quantity, resulting in contamination of the product sulfuric acid through the exchange acid to be supplied to the absorption tower. The presence of mercury, though in a very small quantity, in the sulfuric acid poses a problem from the viewpoint of public nuisance inasmuch as the sulfuric acid is employed as an industrial material in large quantities in various fields of industry. Accordingly, there has been a need for development of a method of manufacturing sulfuric acid which method is effective for removing mercury concurrently.
As a method of manufacturing sulfuric acid by the contact process to meet this demand, the mercury removing treatment method for high-temperature roasting gas disclosed by the Outokumpu OY Patent Application (First Publication No. 2206/1972) is known. According to this method (hereinafter referred to as Outokumpu method), the high-temperature roasting gas coming out of a hot Cottrell is introduced into the sulfatizer, wherein sulfatization is effected to precipitate mercury as a sulfate; the precipitate recovered is washed by water; the iron, zinc sulfates, etc. are separated by virtue of the differences in their solubility in water, whereby mercury, selenium, etc. which are hard to dissolve in water are recovered. On this occasion, the water-soluble substances are returned to the roasting furnace in the form of an aqueous solution. In this connection, the roasting gas (namely, SO.sub.2 gas) having undergone said sulfatization still contains mercury to the extent of about 0.2 mg/N m.sup.3. The gas coming out of the sulfatizer is washed with 30% diluted sulfuric acid by means of a venturic scrubber and thereafter sent to the mist Cottrell of the next process.
The foregoing Outokumpu method is admittedly capable of producing sulfuric acid by efficiently removing mercury from the roasting gas and concurrently recovering the separated mercury. According to this method, however, the temperature of the circulated acid is controlled by an external heat exchanger to prevent the circulated acid from being diluted by absorbing the moisture of the roasting gas on the occasion of sulfatization, and the sulfuric acid enters the sulfatizer at a temperature of 40.degree. C and comes out of the sulfatizer at a temperature of 180.degree. C. The roasting gas is controlled to enter the sulfatizer at a temperature of 350.degree. C and come out of the same at a temperature of 180.degree. C. The temperature of the sulfuric acid and roasting gas is so high that it is difficult to choose an appropriate material for the sulfatizer, heat exchanger, etc. Especially, in case of the presence of fluorine concurrently contained therein, the interior of the sulfatizer is apt to corrode and, therefore, it must be often repaired by suspending the operation of the sulfatizer. Not only that, the dust which is not removed by the hot Cottrell is apt to scatter and react with sulfuric acid in the sulfatizer of the Outokumpu method to produce to ZnSO.sub.4, FeSO.sub.4, CaSO.sub.4 and the like, which may deposit within the pipes when sulfuric acid is cooled and cause such troubles as clogging of pipes and so on.